1. Technical Field
The present disclosure relates to capacitor leakage compensation circuits in Phase-Locked Loops (PLLs).
2. Background Information
The loop filter of a Phase-Locked Loop (PLL) may involve a capacitor having a relatively large capacitance. Realizing this capacitor in integrated circuit form may consume an undesirably large amount of area on the integrated circuit. The capacitor may be implemented in different ways. If the capacitor is implemented in one way, then the capacitor will occupy a larger amount of integrated circuit area but will leak only to a small degree. On the other hand, if the capacitor is implemented in another way then the capacitor may occupy a smaller amount of integrated circuit area but will leak to a larger degree. In some cases, the capacitor is implemented as the relatively small but leaky capacitor, but then a capacitor leakage compensation circuit is provided. The capacitor leakage compensation circuit includes a small version of the leaking capacitor that is referred to as a “replica” capacitor. How the larger capacitor and the replica capacitors leak as a function of voltage is the same, only the replica capacitor leaks less in proportion to how much smaller it is than the larger capacitor. The capacitor leakage compensation circuit operates to maintain a voltage across the replica capacitor that is the same as the voltage across the leaking capacitor of the loop filter when the loop filter is operating. The capacitor leakage compensation circuit, however, detects the current that leaks through the replica capacitor. Because the relationship in size between the replica capacitor and the leaking capacitor in the loop filter is known, the compensation circuit operates to supply a multiple of the detected amount of leakage current onto the leaking capacitor in the loop filter. Ideally, the magnitude of this compensation current is the same as the magnitude of the current leaking through the leaking capacitor of the loop filter. Such a capacitor leakage current compensation circuit allows the capacitor of the loop filter to be realized as the smaller type of capacitor that is relatively leaky, thereby saving integrated circuit area as compared to the size that the loop filter would have were the loop filter implemented using the larger type of capacitor that leaks less. The overall amount of integrated circuit area consumed, the overall amount of current consumption, and the performance of the combination of the leaking capacitor and the compensation circuit should be preferable to the alternative amount of integrated circuit area, current consumption, and performance of the loop filter had the loop filter been implemented with the larger but less leaky capacitor.
A first example of a capacitor leakage compensation circuit is set forth in FIG. 10 of U.S. Pat. No. 6,956,417. This capacitor leakage compensation circuit 190 may be part of a phase-locked loop involving a phase detector, a charge pump, a loop filter, a Voltage Controlled Oscillator, and a loop divider. Resistor RREF and capacitor C1 make up the loop filter. Capacitor C1 is the leaking capacitor. Capacitor C4 is the replica capacitor. The voltage VC is the voltage across the leaking capacitor. The capacitor leakage compensation circuit 190 senses this voltage and supplies a compensation current back onto this same node VC. This circuit is an example of what is sometimes referred to as a “direct sensing” circuit because it senses the voltage on the leaking capacitor itself.
A second example of a capacitor leakage compensation circuit is set forth in
FIG. 4 of U.S. Pat. No. 6,963,232. Reference numeral 54 identifies the leaking capacitor. Current 57 is the current that leaks through capacitor 54. Capacitor 59 is the replica capacitor. The leakage compensation circuit 39 senses the voltage across the leaking capacitor indirectly at the input of VCO 42, places this voltage across the replica capacitor, detects a current 70 that leaks through the replica capacitor, and supplies a multiple of the detected leaking current onto node 67 in the form of a compensation current 58. Compensation current 58 compensates for the current 57 that leaks through capacitor 54 to ground. This circuit is an example of “indirect sensing” in that the leakage compensation circuit senses the voltage on the leaking capacitor indirectly.